Either can help you determine whether you're still claiming the right number of withholding allowances on the Form W-4 that you filled out for your employer when you started your current job, got married or had kids.

But the number you chose at the time may not be the right one for 2018. That's because the new tax law changed the underlying structure on which withholding allowances are based, particularly by eliminating the personal exemption and changing the eligibility for a now-expanded child tax credit.

While it would be a good idea for everyone to double check their allowances, it's especially recommended if you have a more complex tax situation. For instance, you have more than one job or an outside source of income, you normally take a lot of itemized deductions, or you and your spouse both work and have a few kids.

"Withholding issues can be complicated, and the calculator is designed to help employees make changes based on their financial situation," said acting IRS Commissioner David Kautter. "Taking a few minutes can help taxpayers ensure they don't have too little — or too much — withheld from their paycheck."

The goal, he said, is to "avoid surprises" when you file your taxes in 2019.

Most tax filers will want to avoid being "under-withheld" — meaning the number of allowances you used to take is now too high given the tax law changes. In that case, if you don't reduce your withholding allowances for this year, you'll risk owing money to Uncle Sam when you file your 2018 taxes, or at least not getting as big of a refund as you'reused to.

The IRS suggests people at risk of being under-withheld may include those who itemized their deductions in the past, have two or more jobs in their household, or who have dependents age 17 or over. (Note: most wage earners are "over-withheld," meaning they take too few allowances and end up getting a refund come tax time.)